Also i agree with the idea not to show all packages in this window because horizontal scrolling is inconvenient. Tree view might be better.

Further feedback appreciated. Does anyone know how to make the text automatically wrap to the next line in the treeview?

Instead of word wrap why don’t you use the ellipsise property and cut the long lines to the width of the window and display a dialog with a full list when you double click the row?

1. you need to keep the old details pane (except to the right not under) because this summary display will never be sufficient in case of a mass-upgrade with hundreds of changes (such as when updating from one distro release to the next one)

2. Ideally this details pane would present a graphical-diff like view :
sorted list of packages before transaction to the left, sorted list of packages after transaction to the right, with nice colours to show what was added/removed/updated. Perhaps something as simple as passing text versioned package lists to meld would do it
(so it would really be a three-column view: update list, before, after, which would render fine on modern widescreens)

3. And then the obvious enhancement : allow selecting a range of transactions in the history column, and present a consolidated diff view (from the oldest selected transaction to the most recent one) in the details pane

4. It would be nice if a long mouse-over a package name could bring up a tooltip showing the info on this package. Some package names are really obscure, users do not remember what’s behind at first sight

That is not to say the current history is not nice, but your average history user first concern is going to be “what changed exactly between X and Y” and a coloured text-only diff view will communicate this better than pretty icons.